For me the libsigc++ version on slackbuilds.org will not compile (this is on slackware64), however the latest version of libsigc++ (2.2.3) compiles just fine.
I'm just posting this in case others run into this problem, and possibly so it can be fixed.

Edit: I realised this on on -current, and this slackbuild is built for 12.2, so it may not need to be fixed yet.